The Teen Brain: It's Just Not Grown Up Yet

Neurologist Francis Jensen examining a teenage patient. Jensen decided to study the teenage brain when her own sons became teenagers. Now Jensen lectures to teens and parents about how teenagers' brains are different. (NPR)

When adolescence hit Frances Jensen's sons, she often found herself wondering, like all parents of teenagers, "What were you thinking?"

"It's a resounding mantra of parents and teachers," says Jensen, who's a pediatric neurologist at Children's Hospital in Boston.

Like when son number one, Andrew, turned 16, dyed his hair black with red stripes and went off to school wearing studded leather and platform shoes. And his grades went south.

"I watched my child morph into another being, and yet I knew deep down inside it was the same Andrew," Jensen says. Suddenly her own children seemed like an alien species.

Jensen is a Harvard expert on epilepsy, not adolescent brain development. As she coped with her boys' sour moods and their exasperating assumption that somebody else will pick up their dirty clothes, she decided to investigate what neuroscientists are discovering about teenagers' brains that makes them behave that way.

Jensen's older son Andrew Murphy, now a physics major at Wesleyan, is the reason his mother first started studying the teenage brain. She wanted to find out what was causing his maddening teenage behavior.

Teenage Brains Are Different

She learned that that it's not so much what teens are thinking — it's how.

Jensen says scientists used to think human brain development was pretty complete by age 10. Or as she puts it, that "a teenage brain is just an adult brain with fewer miles on it."

But it's not. To begin with, she says, a crucial part of the brain — the frontal lobes — are not fully connected. Really.

"It's the part of the brain that says: 'Is this a good idea? What is the consequence of this action?' " Jensen says. "It's not that they don't have a frontal lobe. And they can use it. But they're going to access it more slowly."

That's because the nerve cells that connect teenagers' frontal lobes with the rest of their brains are sluggish. Teenagers don't have as much of the fatty coating called myelin, or "white matter," that adults have in this area.

Think of it as insulation on an electrical wire. Nerves need myelin for nerve signals to flow freely. Spotty or thin myelin leads to inefficient communication between one part of the brain and another.

A Partially Connected Frontal Lobe

Jensen thinks this explains what was going on inside the brain of her younger son, Will, when he turned 16. Like Andrew, he'd been a good student, a straight arrow, with good grades and high SAT scores. But one morning on the way to school, he turned left in front of an oncoming vehicle. He and the other driver were OK, but there was serious damage to the car.

"It was, uh, totaled," Will says. "Down and out. And it was about 10 minutes before morning assembly. So most of the school passed by my wrecked car with me standing next to it."

"And lo and behold," his mother adds, "who was the other driver? It was a 21-year-old — also probably not with a completely connected frontal lobe." Recent studies show that neural insulation isn't complete until the mid-20s.

This also may explain why teenagers often seem so maddeningly self-centered. "You think of them as these surly, rude, selfish people," Jensen says. "Well, actually, that's the developmental stage they're at. They aren't yet at that place where they're thinking about — or capable, necessarily, of thinking about the effects of their behavior on other people. That requires insight."

And insight requires — that's right — a fully connected frontal lobe.

More Vulnerable To Addiction

But that's not the only big difference in teenagers' brains. Nature made the brains of children and adolescents excitable. Their brain chemistry is tuned to be responsive to everything in their environment. After all, that's what makes kids learn so easily.

But this can work in ways that are not so good. Take alcohol, for example. Or nicotine, cannabis, cocaine, ecstasy ...

"Addiction has been shown to be essentially a form of 'learning,' " Jensen says. After all, if the brain is wired to form new connections in response to the environment, and potent psychoactive drugs suddenly enter that environment, those substances are "tapping into a much more robust habit-forming ability that adolescents have, compared to adults."

So studies have shown that a teenager who smokes pot will still show cognitive deficits days later. An adult who smokes the same dose will return to cognitive baseline much faster.

When Will asked why, his mom could give him chapter and verse on drugs and teen brains. So they would know, she says, "that if I smoke pot tonight and I have an exam in two days' time, I'm going to do worse. It's a fact."

There were other advantages to having a neuroscientist mom, Will says. Like when he was tempted to pull an all-nighter.

"She would say, 'read it tonight and then go to sleep,'" he says. "And what she explained to me is that it will take [what you've been reading] from your short-term memory and while you sleep you will consolidate it. And actually you will know it better in the morning than right before you went to sleep."

It worked every time, he says.

It also worked for Andrew, the former Goth. He's now a senior at Wesleyan University, majoring in physics.

"I think she's great! I would not be where I am without her in my life!" Andrew says of his mom.

For any parent who has survived teenagers, there are no sweeter words.

Transcript

Today in Your Health, we will report on the mysteries of the middle-aged brain. Sure, maybe you're misplacing your keys a little more but in a minute, we'll tell you there's good news, too.

MONTAGNE: But first, we're going to see about what's going on in the teenage brain. When adolescence hits, you may find yourself the parent of a newly frustrating creature - sullen, self-centered and reckless. Or you might've been one at one time. Beneath that brand-new mohawk, there is a brain. It just works differently. One scientist mom decided to find out why. Here's NPR's Richard Knox.

RICHARD KNOX: When her two sons hit adolescence, Frances Jensen found herself saying the same thing over and over again.

Dr. FRANCES JENSEN (Pediatric Neurologist, Children's Hospital, Boston): What were you thinking? You know, this is that resounding mantra of parents and teachers.

KNOX: Jensen couldn't fathom her teenagers' behavior: the surly moods, the exasperating changes of plans, the assumption that somebody else will pick up their dirty clothes.

Now, Jensen's a pediatric neurologist at Children's Hospital in Boston. But her own teens suddenly seemed like aliens.

Dr. JENSEN: I thought, you know, I'm going to try to understand what's going on with this species, you know, these teenagers that we're living with.

KNOX: What really got her into gear was her older son, Andrew. It was when he turned 16.

Dr. JENSEN: He's a very compliant kid, was a great kid. And then his grades started to slip, and this hadn't been an issue before.

Mr. ANDREW JENSEN: I did dumb things. Yeah. I got in trouble with the school sometimes for just not really caring, doing really poorly in classes.

KNOX: He dyed his hair black with red streaks, started going off to school in studded leather and platform shoes.

Dr. JENSEN: I watched my child morph into another being, yet I knew deep down inside, it was the same Andrew.

KNOX: As Jensen dug up studies on teenage brains, she discovered it's not so much what they're thinking, it's how. Jensen says scientists used to think that the teen brain is just an adult brain with fewer miles on it. It's not. To begin with, a crucial part of the brain, the frontal lobe, is not fully connected.

Dr. JENSEN: It's the part of the brain that says: Is this a good idea? What is the consequence of this action?

(Soundbite of laughter)

KNOX: It turns out that the nerve cells that connect teenagers' frontal lobes with the rest of their brain don't work very well. They're sluggish. This is a little complicated, but it's really interesting. Teenagers don't have as much of the fatty coating, called myelin, that adult brains have in this area. Think of it as insulation on an electrical wire. Nerves need myelin for nerve signals to flow freely.

Dr. JENSEN: It's not that they don't have a frontal lobe. And they can use it. They're going to access it more slowly.

KNOX: This might explain what was going on in the brain of Jensen's younger son, Will, when he was 16. He's a straight-arrow student, good grades, high SAT scores. But one morning on the way to school, he turned left into oncoming traffic. He and the other driver were OK, but there was serious damage to the car.

Mr. WILL JENSEN: It was totaled. It was down and out. So it had to get towed away.

Dr. JENSEN: And lo and behold, who was the other driver? It was a 21-year-old -also probably not with a completely connected frontal lobe.

Dr. JENSEN: You think of them as these surly, rude, selfish people. Well, actually, that's the developmental stage they're at. They aren't yet at that place where they're thinking about - or capable, necessarily, of thinking about the effects of their behavior on others. That requires insight.

But that's not the only big difference in teenagers' brains. In children and adolescents, nature made the brain to be excitable, responsive to everything in the environment. That's what makes kids learn so easily. But this can work in ways that are not so good. Take alcohol, for example.

Dr. JENSEN: Addiction has been shown to be, essentially, a form of learning. So they're tapping into a much more robust habit-forming ability that adolescents have, compared to adults.

KNOX: This came in handy in Jensen's own household.

Mr. WILL JENSEN: Most parents, they'll say, don't drink. Don't do drugs. And I'm the type of kid who would say, why?

KNOX: That's Will, son number two. When he asked why, his mom could give him chapter and verse about how drug effects linger for days. So if her boys were tempted, they knew what to say to themselves.

Dr. JENSEN: It's a fact that, you know, if I smoke pot tonight and I have an exam in two days' time, I'm going to do worse. It's a fact.

KNOX: Will says having a neuroscientist for a mom was helpful with his homework, too, like when he was tempted to pull an all-nighter.

Mr. WILL JENSEN: She would say, read it all tonight and then go to sleep. And what she explained to me is that it will take from your short-term memory what you've been reading and what you know now, and while you sleep, it will consolidate it. And actually, you will know it better in the morning than right before, when you went to sleep.

KNOX: He says it worked every time. His older brother Andrew, the former Goth, agrees. He's now a senior in physics at Wesleyan University.

Mr. ANDREW JENSEN: I think she's great. I would not be where I am without her in my life.

KNOX: As a parent myself, who's survived two teenagers, I can tell you there are no sweeter words.